Oxymethylene polymers, as that term is used herein and further defined below, means those polymers having recurring --OCH.sub.2 -- units directly attached to each other. Such polymers have been known for many years. They may be prepared by the polymerization of anhydrous formaldehyde or by the polymerization of trioxane, which is a cyclic trimer of formaldehyde. Oxymethylene copolymers have at least one chain containing recurring oxymethylene units interspersed with --OR-- groups in the main polymer chain, where R is a divalent radical containing at least two carbon atoms directly linked to each other and positioned in the polymer chain between the two valences, with any substituents on the R radical being inert, that is those which are free of interfering functional groups and will not induce undesirable reactions. Particularly preferred are copolymers which contain from 60 to 99.9 mol percent of recurring oxymethylene groups. The R may be, for example, an alkylene or substituted alkylene group containing at least two carbon atoms.
Among the copolymers which may be utilized are those having a structure comprising recurring units having the formula: ##STR1## wherein n is an integer from zero to 5 and wherein n is zero in from 60 to 99.9 percent of the recurring units. R.sub.1 and R.sub.2 are inert substituents, that is, substituents which are free of interfering functional groups and will not induce undesirable reactions.
Particularly preferred oxymethylene copolymers are those having incorporated therein oxyalkylene units having adjacent carbon atoms which are derived from cyclic ethers having adjacent carbon atoms. These copolymers may be prepared by copolymerizing trioxane with a cyclic ether having the structure: ##STR2## wherein n is an integer from zero to 2.
Examples of preferred polymers include copolymers of trioxane and cyclic ethers containing at least two adjacent carbon atoms such as the copolymers disclosed in U.S. Pat. No. 3,027,352, incorporated herein by reference. Among the specific cyclic ethers which may be used are ethylene oxide; 1,3-dioxolane; 1,3,5-trioxepane; 1,3-dioxane; trimethylene oxide; pentamethylene oxide; 1,2-propylene oxide; 1,2-butylene oxide; neopentyl glycol formal; pentaerythritol diformal; paraldehyde; tetrahydrofuran and butadiene monoxide. As used in the specification and claims of this application, the term "copolymer" means polymers having two or more monomeric groups, including terpolymers and higher polymers.
After polymerization, oxymethylene polymers, such as those comprising trioxane-ethylene oxide copolymer chains, contain unstable polyformaldehyde ends, which must be removed in order to improve the thermal stability and other properties of the acetal copolymer. For this purpose, a hydrolysis process, such as that disclosed in U.S. Pat. No. 3,219,623, incorporated herein by reference, may be used. More specifically, a melt hydrolysis process, such as that disclosed in U.S. Pat. Nos. 3,318,848 and 3,418,280, incorporated herein by reference, is preferably utilized.
In the past, it was very difficult to remove enough volatile material (e.g. trioxane, formaldehyde, formic acid, water, hydrolysis agent, etc.) to provide a product suitable for direct use. Subsequent devolatilization and compounding were required to remove volatiles in separate steps, to avoid excessive color formation.
Copending U.S. patent application Ser. No. 664,796 of Auerbach et al, filed Oct. 25, 1984, incorporated herein by reference, describes and claims a moldable oxymethylene polymer containing a minor amount of a polyamide stabilizer which must be in a dispersion. The composition may also contain a hindered phenol antioxidant and an amidine stabilizer.
The incidence of black speck contamination in conventional extrusion or compounding equipment is accelerated when processing molten oxymethylene polymers containing polyamide stabilizers. Under such circumstances it is common to require cleaning the equipment as frequently as every two or three days, to produce oxymethylene molding compositions having an acceptably low level of black speck contamination.
Accordingly, there exists a need in the art for an improved method of devolatilizing oxymethylene polymers containing polyamide stabilizers, to remove formaldehyde and other volatile materials in a manner such that black speck generation is minimized and the need for frequent cleaning of extrusion or compounding equipment is substantially reduced. The process of the present invention provides an answer to this need and also provides a product which has a low level of free formaldehyde, low tendency to form mold deposits, low molding odor, good heat aged color, in addition to a low level of black specks, an does not require the polyamide stabilizer to be added as a dispersion.